An autoinjector is an automatic injection device designed to facilitate delivery of a dose of medicament to a patient through a hypodermic needle, the injection usually being administered by the patient themselves. An autoinjector works, for example, by delivering an injection automatically upon actuation by the patient pressing a button, moving a lever or part of a housing etc. This is in contrast to a conventional manual syringe where the patient themselves needs to directly depress a plunger into a barrel containing medicament in order to effect the injection. The terms “autoinjector” and “injection device” are used interchangeably in the following description.
An autoinjector is described in our international patent application published under number WO 2005/070481. Some of the reference numerals in the present application correspond with the equivalent components in the device described in WO 2005/070481. This device requires that the needle is moved axially so that it can appear beyond the end of the nozzle for the duration of the injection, after which the needle retracts automatically, so that it is never in sight of the user. The device also requires that the plunger is moved axially so that medicament is ejected. The overall complexity of the autoinjector is significantly reduced by both of these requirements being effected by one component, namely an inner housing (illustrated in FIG. 1 of the present application) and the device has the significant advantage that it can be built around a conventional or standard syringe presentation.
The injection device of WO 2005/070481 is designed to be used in conjunction with a standard drug presentation e.g. a syringe comprising a needle, barrel preloaded with medicament and a plunger. There is a significant commercial advantage in being able to use a standard syringe, which will have been subjected to numerous clinical trials, drug stability studies and regulatory approval. Any modification to the standard syringe may require further trials and approval, adding delay and expense. The present invention is relevant to any injection device for use in conjunction with a syringe (whether preloaded or not and whether single-use or reusable), not only the injection device described in WO 2005/070481.
In the known device described in patent application no WO 2005/070481, the syringe is supported within the injection device by a barrel or syringe holder 9. The syringe holder 9 comprises an elongate housing which closely surrounds the glass barrel of the syringe. The annular flange 90 at the rear of the syringe barrel rests on a barrel seat 91 at the rear of the syringe holder 9. The annular flange 90 at the rear of the syringe barrel is often referred to as a “finger flange” because, during a conventional (manual) injection using a syringe, the user's index and middle fingers rest naturally in front of the “finger flange” in order to provide the necessary resistance to allow depression of the plunger by the thumb to deliver the medicament. The barrel seat 91 preferably prevents forward axial movement of the syringe with respect to the syringe holder so that, in use, the syringe barrel and the syringe holder move axially together as one unit.
The inner housing 7 shown in FIG. 1 includes rear tags 7A which can flex radially into and out of contact with the plunger of the syringe and front tags 7B which can flex radially into and out of contact with the finger flange of the syringe barrel.
In use, as described in WO 2005/070481, there are three stages of delivering an injection. Before delivering an injection (referring to FIG. 2 of the present application), the end cap 15 is pulled off, taking the rigid needle cover 17 (if present) and rubber needle sheath with it. In the first stage of delivering an injection, as shown in FIG. 3 of the present application, the tags 7B at the forward end of the inner housing 7 are in contact with the syringe barrel 90, which is pushed axially forward (taking the syringe holder 9 with it), so that the needle 10, which is fixed to the front end of the barrel, moves in the direction indicated by the arrow so that eventually it protrudes beyond the nozzle 11 at the front of the device. Forward travel of the barrel and syringe holder is limited when a surface 9A of the syringe holder reaches an endstop 11A inside the nozzle or front housing 11.
Referring now to FIG. 4, the second stage of the injection is the delivery of the medicament wherein the tags 7A at the rear of the inner housing 7 depress the plunger 8 into the barrel of the syringe. During this stage, the barrel of the syringe is held axially stationary, by abutment of the annular “finger” flange 90 against the barrel seat 91, which results in the barrel being placed in tension as the plunger pushes the non-compressible liquid medicament towards the forward end of the barrel. This tension is undesirable in a glass barrel, which may become damaged or broken, especially if the medicament comprises a particularly viscous liquid which requires greater force to expel it from the syringe via the needle. Viscous medicaments are desirable in certain applications, where the use of a sustained-release viscous medicament reduces the frequency that an injection is required. The undesirable tension on the barrel during injection can be reduced by using an alternative syringe holder 100 which is shown in FIG. 5 and discussed in our UK patent application number 0620163.6. This type of syringe holder supports the barrel at its front end, using for example radially-flexible fingers 108 which have gripping means on the interior surface thereof.
In the third stage of the injection (not illustrated in the present application but shown in WO 2005/070481), once the medicament has been delivered and the inner housing 7 is no longer in contact with the barrel or plunger of the syringe, the secondary spring 12 pushes the syringe holder (and hence the syringe contained therein) axially rearwardly so as to retract the syringe back into the housing so that the used needle is concealed from view.
FIG. 6 shows the device from GB0620163.6, in its storage condition. Note that the flange 90 of the barrel is forward of the front tags 7B of the inner housing 7. Whilst the endcap 15 is in place, none of the internal components of the device can move axially relative to one another.
When the endcap 15 and needle sheath 17 are removed so that the device is ready for use, it is possible that the syringe will have a tendency to move slightly axially with respect to the outer housing and the inner housing 7. This undesired axial movement could be as a result of “bounce-back” caused by the sudden release of the needle sheath from the needle, or could depend upon the orientation in which the user is holding the device, for example.
This undesired axial movement of the syringe could be prevented by storing the tags 7B in a radially-inward position, i.e. already abutting flange 90 so that it is impossible for the syringe to move rearwardly with respect to the inner housing 7. However, this solution is undesirable because, after storage for a prolonged period of time, it is possible that the plastic tags 7B will acquire “memory” of their radially-inward position such that, during operation of the device, they are not capable of springing radially-outwardly when required to be out of engagement with the syringe barrel (i.e. during the third stage described above).
The performance of the radially-flexible tags 7B during delivery of an injection is optimised by storing them in a radially-outward position (as illustrated in FIG. 6) so that they can be temporarily forced inwardly into contact with the flange 90 when required to move the syringe and its attached needle forward for delivery of an injection, after which they can easily spring back into the radially-outward position (of which they have acquired “memory”) after injection when it is desired for the syringe to retract fully into the housing. However, storage of the tags 7B in a radially-outward position means that there is nothing to prevent the undesired axial movement mentioned above when the endcap 15 and needle sheath 17 are removed.
If the undesired axial movement is rearward such that the flange 90 of the barrel comes to rest in a position rearward of the tags 7B, it will not be possible for the device to operate properly because the tags 7B will be unable to engage with the flange in order to move the syringe axially forward as is necessary during the first stage of the injection described above. The failure of the device to operate properly would only become apparent upon actuation of the injection causing uncertainty for the patient and risk to health resulting from incomplete or no dose of medicament being delivered.
A second disadvantage of the prior art arises due to variations in the initial absolute axial position of the rear flange of the plunger from one syringe to the next. The rear flange of the plunger needs to be engaged by the rear tags of the inner housing in order to deliver an injection. These variations in initial axial position could, for example, be caused by manufacturing tolerances, variations in the filling of the syringe with medicament, variations in the volume of the gas bubble inside the syringe, changes in atmospheric pressure (for example if the device is transported by airfreight) etc. These slight variations may affect the volume of the dose of medicament being delivered. The dose of medicament to be delivered is determined by the extent of the axial movement of the plunger into the syringe barrel, which in turn is controlled by the relative positioning of the tags 7A, 7B on the inner housing 7. For a given axial distance travelled by the inner housing (and the rear tags thereof), the actual dose of medicament delivered may vary slightly depending upon the absolute axial starting position of the plunger. This problem is particularly relevant in devices which are designed to deliver a partial dose (e.g. half or quarter) of a full syringe, compared with devices which are designed to deliver a full dose (i.e. completely emptying the syringe). The absolute axial starting position of the plunger may therefore be of importance in maintaining the accuracy of the actual dose of medicament delivered.